A typical structure of polymer light-emitting diodes (PLEDs) consists of a hole injection electrode (anode), a layer of light-emitting polymer (LEP) and an electron injection electrode (cathode). Usually the anode layer consists of a transparent conducting film such as indium-tin-oxide (ITO) with a layer of conducting polymer, such as poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulphonate) (PEDOT:PSS). The purpose of the PEDOT:PSS layer is to improve hole injection into the LEP by increasing the work function of the injection layer and providing a better physical contact between the LEP and the injection layer. The polymer layers are typically spin coated, though advanced printing methods can also be employed. The cathode layer is typically a layer of low work function metal, such as Ba or Ca, capable of effectively injecting electrons into the LEP layer, capped with a layer of another metal such as Al.
The color of light emission from such a device structure is controlled by emission properties of the LEP layer. For example, emitting polymers such as PPV and MEH-PPV emit in the green and orange, which corresponds to the band gap of the respective polymers. Broad spectrum emission such as white emission can be achieved by blending a blue-emitting LEP with polymers (or small molecules) that emit in green and red regions of spectrum. In this case direct carrier trapping and/or energy transfer from the blue host to the red and green dopants will redistribute emission between blue, green and red chromophores thus resulting in white emission. A similar approach is to synthesize a copolymer incorporating all three types of chromophores in one polymer chain thus preventing possible phase separation that may occur in a blend.
In order to optimize the PLEDs, both the device structure and material set needs to be optimized to obtain good efficiency and reliability. This is relatively simpler to do for single color emitting PLEDs than for broad spectrum PLEDS for the following reasons: (1) Since only very small concentration of the emitting dopants are required to change the color of emission, the tolerances of the concentrations of these dopants in the host LEP have to be very tight in order to have sufficient reproducibility. (2) In addition to affecting the color, changing the concentrations of the emitting dopants, or changing the dopant can also result in undesirable changes in charge transport (e.g. trapping of charges) properties of the host LEP which can adversely affect device performance. (3) The stability of these emitting chromophores in the host and in the presence of each other across the operational life of the device is also an issue as illustrated in FIG. 1. What is optimum for one emitter, is usually not optimum for the other emitters as illustrated in the degradation patterns shown in FIG. 1. An alternative to the above issues is to inkjet print the LEP layer which suffers from complicated processing issues of its own. Thus, there is a great need for a method to obtain broad spectrum PLEDs that do not suffer from the issues stated above.